1. Field of the Art
This invention relates to a method and an apparatus, for bonding substrate plates face to face in a predetermined gap relation through an interposed spacer material, and more particularly to a method and apparatus for bonding, for example, upper and lower substrate plates which form a liquid crystal cell of an LCD panel, strictly registering relative positions of the two substrate plates with each other and then pressing the substrate plates toward each other to compress and flatten the interposed sealer material to such a degree as to form a gap of a predetermined width between the upper and lower substrate plates.
2. Prior Art
A liquid crystal cell of an LCD panel is formed by bonding together a couple of substrate plates of transparent synthetic resin material through a sea sealer material which is interposed between and along outer marginal edges of the substrate plates, and filling liquid crystal in a sealed gap space which is formed between the two substrate plates by the sealer material. For instance, in the case of a color TFT type LCD panel, a lower substrate plate constitutes a TFT substrate which has circuit patterns including TFT elements formed thereon in the fashion of a matrix, while an upper substrate constitutes as a filter substrate which has color filters formed thereon A driver IC and a printed circuit board are connected to the TFT substrate of the LCD panel. More particularly, the LCD panel is fabricated in the manner as described below.
Firstly, a sealer material is applied along outer marginal edges of either the TFT substrate or the filter substrate. Then, the two substrate plates are overlapped one on the other, namely, in such a way that the filter substrate comes on top of the TFT substrate. Normally, a sealer material is applied on the side of the lower TFT substrate and in the shape of a rectangular frame which is partly opened by a void portion. The initial thickness of the applied sealer material is larger than the width of a gap space to be ultimately formed between the two substrate plates in a subsequent hot-pressing stage.
In the next place, the two substrate plates are adjusted to a registered position relative to each other and the overlapped substrate plates are pressed and temporarily fixed to each other. At this time, it has been the general practice to apply, for example, an ultraviolet hardening synthetic resin spot-wise and to irradiate ultraviolet ray for temporarily fixing them. Thereafter, the substrate plates are pressed toward each other under heated conditions in such a way as to press and flatten the interposed sealer material for adjusting a gap width between the two substrate plates. In this hot-pressing stage, heat is applied to harden the sealer material. However, the sealer material is hardened only to such a degree as to maintain a certain gap width between the two substrate plates in a stable state. Finally, the temporarily bonded substrate plates are sent to a hot-hardening stage to thermally harden the sealer material completely.
In the above-mentioned hot-pressing stage, a couple of substrate plates are pressed under heated conditions by the use of a press means, which is provided with heated upper and lower press members. However, since the substrate plates of an LCD panel are formed of a relatively fragile material like glass and synthetic resin, they can be easily damaged if hot press members are abutted directly against the respective substrate plates. Therefore, in most cases the abutting faces of the upper and lower pressing members are covered with a cushioning material like a rubber sheet of a predetermined thickness, so that the pressing members are abutted against the substrate plates through a cushioning material.
In this connection, recently there is a strong trend toward high-resolution fine-pitch LCD panels, which require to register and bond substrate plates to each other in an extremely strict and precise manner because a very slight positional deviation of several micrometers or less is impermissible in the case of fine-pitch LCD panels. Therefore, in a substrate bonding stage, positions of substrate plates are adjusted relative to each other very strictly. However, no matter how strictly substrate plates are one on the other before pressing, it cannot guarantee that the bonded substrate plates stay in the registered position without deviations therefrom. The reason for this is that, as the substrate plates are pressed in the hot-pressing stage, a pressing force is applied thereto through a cushioning material which is compressed and flexed under the pressing force. As a cushioning material is compressed and caused to flex, it is difficult to control the direction of flexure, which more or less differ from one cushioning member to another. Namely, in some cases the pressing members fail to press substrate plates in a normal straightforward direction, and, since the interposed sealer material is not yet hardened in the hot-pressing stage, may give rise to a detrimental positional deviation between upper and lower substrates. Although the overlapped two substrate plates are temporarily bonded by an ultraviolet hardening synthetic resin, the bondage by the synthetic resin is strong enough only for preventing deviations of the substrate plates in handling or in the course of transfer. If the overlapped substrate plates are strongly bonded by an ultraviolet hardening synthetic resin, it will become difficult to determine the gap width precisely later in the hot-pressing stage.
No positional deviations occur to the overlapped substrate plate as long as the cushioning members on the upper and lower pressing members of the press means of the same compressive characteristics, namely, as long as the cushioning members on upper and lower pressing members are flexed in the same direction and to the same degree and distorted in the same manner when compressed. On the other hand, for example, in case upper and lower cushioning members are flexed in the opposite directions upon compression in the hot-pressing stage, this difference in compressive characteristics results in amplification of relative movements of upper and lower substrate plates and thus in augmentation of positional deviations of the upper and lower substrate plates.
Accordingly, it is important to carefully select suitable cushioning material to be adhered on the pressing members of the press means to make sure that the cushioning pad members on the upper and lower pressing members have the same compressive characteristics. For this reason, the selection of cushioning material is met by various limitations. In this regard, it is possible to produce a large number of cushioning pad members and select therefrom pairs of matching compressive characteristics for the upper and lower pressing members. However, the pad selection of this sort is very troublesome, in addition to a detrimental drawback that a large amount of material has to be wasted. Besides, there is little possibility of finding a pair of cushioning pads which perfectly match in compressive characteristics. Therefore, it is difficult to cope with the demands for fine-pitch LCD panels of higher grades, simply through selection of cushioning pads. Further, since a pressure is applied on substrate plates under heating conditions, cushioning pads undergo accelerated deteriorations under the influence of heat and need to be replaced after use over a certain period of time. The replacements of cushioning pads involve the same problem as explained above.
Thus, there are great limits to preventing positional deviations of substrate plates in a hot-pressing stage by matching compressive characteristics of cushioning pads to be adhered on upper and lower press members. Regarding methods without using cushioning pads, there have thus far been proposed various methods, for example, a method of pressing substrate plates by the use of a fluid pressure. However, since a pressing method of this sort invariably requires complicate equipments for the supply of a fluid and for the control of fluid pressure, it is still the general practice to resort to a pressing method using cushioning pads.
In view of the foregoing situations, it is an object of the present invention to provide a method and an apparatus for hot-pressing substrate plates precisely in an aligned state despite the use of press members with cushioning material.
It is another object of the present invention to a method and an apparatus which can effectively correct positional deviations which normally inevitably occur to upper and lower substrate plate in the course of hot pressing.
It is still another object of the present invention to provide a method and an apparatus which can obviate the necessity for adjusting compressive properties of cushioning material to be bonded on abutting surfaces of upper and lower pressing members, while guaranteeing to hot-press substrate plates accurately.
In accordance with the present invention, for achieving the above-stated objective, there is provided a method for bonding two substrate plates together through a sealer material and in a precisely aligned state in a stage of hot-pressing the two substrate plate to form a gap space of a predetermined width therebetween, the method comprising the steps of: registering two substrate plates one on the other in a registering stage in reference to alignment marks provided on the respective substrate plates and in such offset positions as to cancel positional deviations which predictably occur to the substrate plates during a subsequent hot-pressing operation; provisionally pressing the registered substrate plates through a sealer material in a provisional press stage to form a joined substrate assembly; hot-pressing the joined substrate assembly on a press means and under heated conditions in a hot pressing stage thereby compressing and flattening the sealer material to form a gap space of a predetermined width between the substrate plates and at the same time thermally hardening the sealer material; and
inspecting the hot-pressed substrate assembly in an inspecting stage to check for positional deviations between the upper and lower substrate plates.
In short, in order to correct positional deviations which occur to the upper and lower substrate plates of a joined substrate assembly in the course of a hot-pressing operation, the two substrate plates are set in such offset position in a preceding registering stage as to cancel the predicted relative positional deviations at a hot press station. In this instance, in the registering stage, relative positions of the two substrate plates are either shifted horizontally in two directions along two perpendicularly intersecting axes or shifted in a rotational or angular direction in addition to shifts along two perpendicularly intersecting axes.
According to the present invention, there is also provided an apparatus for bonding two substrate plates together through a sealer material and in a precisely aligned state in the course of hot-pressing the two substrate plate to form a gap space of a predetermined width therebetween, the apparatus comprising: a provisional press mechanism for provisionally registering and bonding the substrate plates into the form of a joined substrate assembly, the provisional press mechanism including upper and lower substrate holder means adapted to hold upper and lower substrate plates in face to face relation, an alignment mark detection means adapted to detect relative positions of the substrate plates from picture images of alignment marks provided on each one of the substrate plates, and a substrate position adjusting means provided in association with one of the substrate holder means for adjusting position of one substrate plate relative to the other substrate plate in reference to the alignment marks; a hot press mechanism located at a hot press station and having upper and lower press members with heating means, the upper and lower press members being movable toward and away from each other for hot-pressing the joined substrate assembly through cushioning material; a positional deviation detecting mechanism adapted to detect relative positional deviations between the two substrate plates of the hot-pressed substrate assembly in reference to the alignment marks; and a control means adapted to record offset data for correction of positional deviations of the substrate plates of the joined substrate assembly occurring in the course of hot pressing, and to control the substrate position adjusting mechanism of the provisional press to set the substrate plates in such offset positions as to cancel predicted positional deviations at the hot press station.
The above and other objects, features and effects of the present invention will become apparent from the following particular description of the invention, taken in conjunction with the accompanying drawings which show by way of example a preferred embodiment of the invention. Needless to say, the present invention should not be construed as being limited to particular forms shown in the drawings.